The field of catalytic cracking and particularly fluid catalyst systems has been in a constant state of development since conception of fluid systems. Thus, as new product demands increased and experience was gained in operating and design parameters, so also were new catalyst concepts developed which further increased needed refinements in cracking technology. With the development of high activity zeolite-type cracking catalysts, the petroleum refiners once again found themselves in need of developing new operating technology. The present invention is concerned with a combination operation relying upon distinct catalyst particles differing in particle size, activity, selectivity and coke producing characteristics, mutually contributing to the catalytic upgrading of separate, relatively high boiling hydrocarbon fractions varying considerably in coke producing characteristics and/or catalyst fouling characteristics.